XML is a markup language that can be used to represent data organized in accordance with a schema that specifies an arbitrary hierarchical structure. In particular, the schema specifies a hierarchy of tags, each representing an element that is capable of having a value and/or values for attributes associated with the element.
It is typical for the designers of a distributed application to specify a schema for organizing the specific data that is used by the application. This schema is shared by all of the application's components. In order to exchange data between such components, a sending component uses its copy of the schema to generate an XML document, or “message” containing the data to be sent, organized using tags in a structure that is consistent with the schema. When the sending component sends the message to a receiving component, the receiving component uses its copy of the schema to understand the structure of the transmitted data, so that it can process and/or store the transmitted data.
An XML document contains tags whose organization (i.e., whose relative embedding) establishes an XML tree structure. Each tag corresponds to a type of node in the XML tree called an element, which may have a value specified in connection with the tag. The XML tree structure may also have attribute nodes, each established as the child of an element node and containing a value for an attribute of that element that is specified in connection with the tag. The first-occurring tag corresponds to a node called the root node. The nodes other than the root node each have one parent. A node can be identified by its “Xpath”: an ordered recitation of the names of the node and all of its parents, beginning with the root node and ending with the identified node.